STRIKE SLIP IN THE SOUTHERN COAST MOUNTAINS - CASCADES OROGEN: THE ROSS LAKE FAULT SYSTEM AND IMPLICATIONS FOR “BAJA BC”

The >10-km-wide Ross Lake fault system (RLFS) is part of a 500-km-long zone of Paleogene, NW-striking high-angle faults at the southern end of the Coast Mountains-Cascades orogen. The RLFS and the Pasayten fault to the east lie near the paleomagnetically defined boundary of the proposed far traveled “Baja BC”, and were active during the appropriate interval for major translation of this block. The RLFS consists of two major sets of faults, both of which record components of strike slip and dip slip. Movement initiated by 65 Ma, and the system records the transition at ~57 Ma from regional transpression to transtension. The eastern fault set bounds the Jura-Cretaceous Methow-Tyaughton basin and in part separates it from its metamorphic equivalents. The western fault set separates the lower-grade, metamorphosed Methow rocks and Bridge River terrane from amphibolite-facies rocks of the North Cascades crystalline core. The latter includes higher-grade equivalents of the Bridge River terrane (Napeequa unit) and potentially the Methow strata. The fault sets merge southward to form the Foggy Dew fault zone where sillimanite-bearing mylonites are placed against sub-greenschist-facies rocks. The fault zone is truncated to the SE by the 49 Ma Cooper Mountain batholith, which also obliterates the intersection of the RLFS with the southern continuation of the Pasayten fault. On strike south of this intrusion, dextral slip was apparently taken up by narrow, discontinuous shear zones. Large strike-slip displacements are precluded by the continuity of E-W-trending contacts and structures, and pre-100 Ma intrusive rocks. These enigmatic relationships imply that slip decreased dramatically along strike and/or is transferred westward to the dextral Entiat fault in the Cascades core. Any transfer must occur on a detachment below the present exposure level, as narrow, steeply dipping Cretaceous plutons are not offset across the stepover. Collectively, tectonostratigraphic ties across the Entiat fault and RLFS, particularly metamorphosed Bridge River terrane, argue against many 100s of km of slip on these structures and the southern Pasayten fault. Thus, although these faults are regionally important, they were probably not the dominant contributors to postulated large translation of Baja BC.